1. Field of the Invention
This invention relates to measurement of characteristics of a semiconductor, especially of the life time of carriers thereof.
2. Brief Description of the Prior Art
The manufacturing processes of semiconductor elements, for example, power diodes or thyristors include cutting a bar-shaped N-type silicon starting substrate to a desired thickness (for example, of the order of several hundred microns) and diffusing a P-type impurity such as galium into a cut N-type silicon wafer to produce a PNP wafer. However, after such PNP wafers are prepared, the life time of carriers often reduces to below a standard value, thereby providing poor or bad wafers.
For this reason, it is necessary to measure the life time of carriers of PNP wafers for the purpose of picking out poor wafers.
A diode method has generally been used for measuring the life time of carriers. In accordance with the diode method, several sheets of PNP wafers are picked up from several tens of sheets of PNP wafers and one of the P-type layers of a PNP wafer is removed by etching to form a PN junction, that, is a diode. Thereafter, a forward current of square waveform is passed through the diode and the life time of carriers is determined from the variation with time in a voltage drop across the anode and cathode of the diode after the interruption of the forward current. In this method, however, even when only one of the picked-up PNP wafer of semiconductor samples has life time below a standard value, the total number of PNP wafers of that particular lot are condemned and thus an improvement in productivity cannot be expected. This method has further a disadvantage that a destructive measurement of PNP wafers prevents the measurement for total number of samples, resulting in an inaccurate measurement.
A method capable of measuring the characteristics of the total number of samples non-destructively has been proposed wherein a sample is inserted into a microwave-guide and a microwave is utilized for the measurement. In this method, however, measurement of characteristics of semiconductors such as diodes and thyristors is impossible since these semiconductor samples have a thickness of about 200 to 1000 microns and a diameter of about 10 to 50 millimeters. Because of large size, they cannot be inserted into the waveguide.